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Thread: Max fuel economy for the 6.5TD

  1. #61
    Join Date
    Apr 2004
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    Virginia
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    127

    Post

    Good to know I am not alone. I also have 3.73s, but I have new injectors. My next step is water injection because I can feel a big difference on warmer days. I have Heaths turbo master? (spring loaded waste gate control) set at stock levels, but I still think I am loosing power from intake temps.
    Eric
    '94 4x4 2500 6.5TD Suburban
    http://sofadog.net/6.5TD/ (work in progress like the truck)

  2. #62
    Join Date
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    Things like engine rpm, vehicle weight & drag, and driving strategies for max fuel economy will apply to just about any light diesel engine. Our vehicle choices affect weight and drag, and have an effect on fuel efficiency.

    I think you'll be surprised at what components are available for all 6.2/6.5 diesels (EFI and MFI) that were designed to improve fuel economy, and we'll be discussing all of them here. Some components you may already know about, but there are a few you probably don't… At least not yet!

    Jim
    Last edited by More Power; 10-09-2018 at 09:32.

  3. #63
    ogrice Guest

    Post

    The Max MPG project uses a 72 El Camino. I have a 56' Ranchero that I wish to put a 6.5td or a 7.3psd in. Under the hood is a 352 FE, and the car has been sitting for the last 20+ years in the garage. Big project, a full restoration, but i'm going to do what it takes to get it back on the road.

  4. #64
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    In reading here I see a lot mentioned about back pressure. If your engine is fighting back pressure you need to address it. The muffler in stock rigs is most likely a big part of this as well as the catylitic converter. Now if you live in an area that monitors this stuff you cant remove it but there are better units that will flow better. Take for example the big trucks, we run 500 HP caterpillars in our big trucks and they breath out through a 5 inch main from the turbo into a Y pipe and split to two 5 inchers up the stacks and out the top. There is very little back pressure here at all.
    If you are seeing more back pressure than boost you need to be opening up the exhaust to let the little beastie breath. The retained heat alone is not doing the engine any good.
    Just a thought.
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2005 Suburban (Papa Smurf)
    THIS IS BOW TIE COUNTRY

  5. #65
    Marty Lau Guest

    Post

    How about adding AIRTABS, http://www.airtab.com/
    they claim a 4-8% increase of MPG with them. If your getting 25MPG that is a full MPG increase. I been think about adding this to my truck for
    S's&G's.

  6. #66
    Truckie117 Guest

    Post

    Hey Guy's
    I am jelous of the milage. I run stock and I mean really stock.I calculate milage every time I fill up and usally get 15.5 to 16.5 for all around diriving hiway street NY bumper to bumper 495 stuff. The best milage I got was when I drove the truck back from VA dealership 20mpg 2 Pump chages and lota miles later 16.5 is the best I can do at fastest 65mph 2000rpms. Would like to up this with the price of fuel.

  7. #67
    Join Date
    Nov 2005
    Location
    Bismarck, ND
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    19

    Default Long time subsciber, first time poster!!

    The only thing left out of the discussion about the affects of faster travel speed... is the simple fact that most of these vehicles are geared to run somewhere well above the optimum 1800 rpm and as you increase travel speed you also increase engine speed. Since the engine efficiency drops off quickly as engine speed increases above 1800 rpm, the faster travel speeds really hurt. A simple test would be to compare fuel economy at the same engine rpm in direct vs. overdrive. That would show you the areo and friction stuff, excluding the engine and some of the transmision. I have lots of mpg data on a stock 1995 1/2 ton 4wd 6.5 turbo with 3.42's. I couldn't tell you what the mileage is at 65 mph, I've never done that! My year around average is about 14.5 mpg. Less when towing (11 ish) and more when not towing, especially at 70-75 mph (16.5 ish). Enuf for my first post.
    1995 GMC Sierra 6.5L K1500 Ext Cab Short Box, 316,000 miles, Kennedy Heart Beat @ 233k miles (21:1), GM Dual Thermostats - Housing - High Flow Water Pump, 9-Blade Steel Fan, Kennedy Diesel Special Calibration Fan Clutch, Kennedy Diesel Exhaust - 3" Downpipe - 3.5" Pipe - See Thru Muffler, DSG Cam Phazer, Kennedy Remote FSD Behind Drivers Side Headlight, A-Team Turbo, Kennedy Diesel Fuel Economy Programming, A-Pillar EGT, Boost & Tranny Temp, GU6 3.42 & 245-75-16 Rubber, Stock GM Flat Air Filter & Box, Zero to 60 mph in 9.5 seconds on Wintermaster.

  8. #68
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    Feb 2000
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    Montana
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    Welcome to the board!

    I've long held that 1800-RPM and 65-MPH are magic numbers for best fuel economy. The engine's torque peak will appear at 1800-1900 RPM and wind resistance at 65-MPH is still manageable, yet allow you to travel at a realistic speed.

    Jim

  9. #69
    Join Date
    Jul 2005
    Location
    LI, NY
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    Makes sense to me with 3.42's and 265's i'm in that 1800 to 2000 rev band and it feels real sweet right there. I see somewhere around 18 - 19mpg, i should point out that I do alot of long distance driving and my foot is not made out of lead. I am just installing a 4" exhaust and hoping to maybe break the 20mpg barrier. Not sure if my foot will stay unleaded tho!

    To me if I had the money an underdrive with my current setup would be a sweet rig that covers all bases. If I had that kind of money a Duramax would probably make more sense tho.

    cheers
    Nobby
    97 K2500 Suburban, 3.42, 245 Tires, Custom Oil cooling lines, Snorkel Removed and 4" Heath Exhaust and Crossover. TSM Rear Disc Conversion.

  10. #70
    Join Date
    Nov 2006
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    Kelowna, BC
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    Default Tire Size

    There was a lot of discussion about how bigger tires cut down milage. I agree that with a typical small gasser engine, this would be the case. However, the diesel puts out so much torque that I don't feel it affects milage. I am running the 36" Military mudders combined with a 4.10 rearend and a NV4500. My truck weighs around 6300 lbs with a full tank of diesel and driver. In town my milage sucks, (15-16). On a recent trip on the highway we kept very close tabs on the fuel consumption. With around 800lbs in the box I got close to 24 mpg. (Imperial) I kept engine speed in between 1,600 and 1,800 rpms which worked out to between 60-65 mph. This is significantly better than with the stock size tires.
    1993 HD2500- 4X4, Nv4500, rc/lb, Lots of mods, killed her. Awaiting her TT rebuild!

    2002 Camaro L36/M49- Killed In Action

    1995 HD2500 - 4X4, NV4500 rc/lb, GL4, Turbo, exhaust

    1994 HD2500- 4X4, NV4500, ec/lb

  11. #71
    Join Date
    Oct 2008
    Location
    Michigan
    Posts
    83

    Default

    I would like to see a manual lockout hub made for the K series vehicles. I feel like this would help mpgs
    1996 GMC Suburban, 6.5 TD, 4L80E, 3.73 Full floating limited slip rear end, 285/75R16E Dayton Timberline ATII's.

  12. #72
    Join Date
    Apr 2001
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    Newberg Oregon
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    The very small amount of stuff thats moving when the rig is not in 4x4 is so small that I doubt you would see any difference outside of a high tech lab.

    The front end is disengaged by an actuator so the gear set and the front drive shaft are stopped under normal conditions.

    The design of the front end (IFS) is not one I am impressed with but they seem to last fairly well when not subjected to nasty poundings.
    The axle design is not readily adaptable to a "Locking hub" design.

    Best

    Robyn
    Last edited by Robyn; 12-10-2008 at 07:36. Reason: addition
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2005 Suburban (Papa Smurf)
    THIS IS BOW TIE COUNTRY

  13. #73
    Join Date
    Jan 2001
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    New Hampshire - Live Free or Die
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    Default

    Interesting. I don't think it'd help mileage much, but I've had 2 of these puppies wear the carrier in the area of the left side gear to the point where the stub axle moved around so much it couldn't seal any longer. A disconnect on both sides would stop the mad spinning of the side gears and spider gears...
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC, John Deere X758
    New ride: MB GLS450 - most stately
    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

  14. #74
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    Apr 2001
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    Newberg Oregon
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    I have always had little love for this design because of these factors.

    The engineers must have been pressured from higher management to make this quicky solution.

    The whole design is just dripping with the telltale signs of "Shortcuts"

    The cheezy single bearing on the wheel end of the axle stub. The thermal actuator and of course the issue spoken of with the disconnect and spider gears.

    Fords articulated rendition of the straight axle was another abortion.

    My feeling is this, if someone just can't deal with a truck, then buy a car and be done with it.

    I have put a couple million miles in behind the wheel of a 4x4 of one type or another with a straight axle up front and never found these to be objectionable.

    Never once did I find myself annoyed at having to get out and turn the hubs in.

    All the old stuff just worked and the mileage was as good as it could be.

    Little 82 K Blazer with a 6.2 that got 24 MPG hwy.

    OMG what were they thinking. ALL the complexity and extra parts and goodies that wear out and need service.
    And to top it off the mileage went right in the toilet too.

    The later trucks are what they are and I dont think there is really much that can be done to improve the mileage a whole lot.

    A 2500 K Burb (IFS) with 6.5 can do around 20 MPG give or take a bit when things are right and driven accordingly.



    best

    Robyn
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2005 Suburban (Papa Smurf)
    THIS IS BOW TIE COUNTRY

  15. #75
    tank140 is offline Landrover defender 140 GMC 6.5 V8 turbo diesel
    Join Date
    Jul 2009
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    Warrington ENGLAND
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    Default high milage

    hi all

    I get about 30 mpg from my Landrover Defender 6.5 TD on the high way over here in the uk
    I think that is great as some of my buddys ony get about 20mpg from there late defenders

    cheers
    tank

    Last edited by tank140; 09-02-2009 at 08:40. Reason: add photo
    Landrover 142 defender GMC 6.5 turbo diesel
    rear mounted turbo
    Loads of GM engine parts all over my garage

  16. #76
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    Newberg Oregon
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    30 is a good number

    The size and weight of the rig makes the difference for sure.

    Lookin good

    Robyn
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2005 Suburban (Papa Smurf)
    THIS IS BOW TIE COUNTRY

  17. #77
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    Sep 2007
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    Jax Florida
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    Quote Originally Posted by tank140 View Post
    hi all

    I get about 30 mpg from my Landrover Defender 6.5 TD on the high way over here in the uk
    I think that is great as some of my buddys ony get about 20mpg from there late defenders

    cheers
    tank

    Is that U.S. gallons? Very nice looking truck.
    93 GMC Sierra 2500
    6.5L TD 5 spd Manual trans Sold

    97 Chevy Silverado CrewCab K3500 Dually
    6.5L TD Auto


  18. #78
    tank140 is offline Landrover defender 140 GMC 6.5 V8 turbo diesel
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    Hi mate

    That is UK Gallon 1UK gal = 1.2 US gal so that would make about 36 us mile / gal

    not bad for a for a 2.5 tonne truck i think

    tank
    Landrover 142 defender GMC 6.5 turbo diesel
    rear mounted turbo
    Loads of GM engine parts all over my garage

  19. #79
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    Quote Originally Posted by tank140 View Post
    Hi mate

    That is UK Gallon 1UK gal = 1.2 US gal so that would make about 36 us mile / gal

    not bad for a for a 2.5 tonne truck i think

    tank
    Actually, it's about 25 MPG/US (30 x .8327 = 24.981). Still, not bad for a 2.5 tonne truck. 36 does sound better, though.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  20. #80
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    Sep 2002
    Location
    Alberta
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    I was reading once over on the TDR(love my cummins) that the efficiency was best served not at a certain amount of boost, speed, or rpm's, but rather by driving with the pyro @ 600 degrees. Apparently some studies have proved that diesel engines are most thermally efficient at that temp. Now if one could combine 60 mph, 1800rpm, and 600* on the pyro...

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